The ground speed of a work vehicle, such as a tractor or other agricultural vehicle, is primarily determined by the set engine speed (i.e., engine rpm, which is referred to herein as “erpm”) and the set transmission gear (referred to herein as “gear ratio” or simply “gear”). When a work vehicle is operating in a manual speed control mode, the erpm is typically set by an erpm throttle lever and the gears are typically set by up/down shift buttons. Thus, to maintain a desired ground speed, the operator must manually adjust both the gear setting and the throttle lever position.
To simplify operator control and to increase the efficiency (e.g., fuel efficiency) of a work vehicle, automatic speed control modes have been developed that allow both erpm control and gear ratio control to be integrated together. For example, U.S. Pat. Pub. No. 2010/0145581, (filing date of Dec. 8, 2008) entitled “Automatic Productivity Management Control with Standard Power Shift Transmission” and assigned to CNH America LLC, discloses a work vehicle with an automatic speed control mode in which an electronic controller may be utilized to make both erpm changes and gear changes based on one or more inputs received by the controller (e.g., vehicle loads, actual ground speed, brake position, throttle position, vehicle slippage and/or the like) in order to maintain the work vehicle at a desired ground speed. Using such an automatic speed control mode, if the load on the work vehicle increases, the controller may automatically lower the gear setting and increase the erpm to accommodate the increased load while still maintaining the desired ground speed. Similarly, if the load on the work vehicle decreases, the controller may automatically increase the gear setting and decrease the erpm in order to increase efficiency and still maintain the desired ground speed. Such an ability to automatically adjust the gear setting and erpm may allow the engine in a work vehicle to continuously operate at or near its maximum torque curve, thereby maximizing its efficiency. The disclosure of U.S. Pat. Pub. No. 2010/0145581 is hereby incorporated by reference herein in its entirety for all purposes.
While the automatic speed control mode described above provides numerous advantages, improvements are still desired to further enhance the overall operation of a work vehicle operating in such a control mode. For example, when it is determined that both an erpm change and a gear change is needed to maintain a desired ground speed, the controller of a work vehicle operating in a conventional automatic speed control mode is configured to simultaneously transmit control commands to the engine and transmission in order to implement such changes. However, due to the differences in system responses to the control commands, the erpm and gear changes are often initiated at different points in time and/or occur at different rates. As a result, the ground speed of the work vehicle momentarily increases or decreases by a significant degree, thereby causing the motion of the vehicle to feel jerky or rough. Unfortunately, given that most work vehicles operate under continuously changing load conditions, the erpm and gear settings of such vehicles must be changed frequently, resulting in recurring and sudden jerky vehicle motion.
Accordingly, a system and method for synchronizing erpm and gear changes while a work vehicle is operating in an automatic speed control mode such that the motion of the vehicle is smoothed during such changes would be welcomed in the technology.